In an upper nozzle adapted to be fitted into a discharge opening of a tundish or a ladle and formed with a bore for allowing molten steel to flow therethrough, alumina and other inclusions are apt to be attached inside the bore to form a deposit thereon, which narrows a flow passage to hinder a casting operation, or is likely to fully clog the flow passage to preclude the casting operation. As one example of a technique for preventing the deposit formation, it has been proposed to provide a gas injection port to inject an inert gas (see, for example, the following Patent Document 1 or 2).
However, an upper nozzle disclosed in the Patent Document 1 or 2 is a gas injection type, which needs to take a lot of time and effort for production due to its complicated structure, and requires an inert gas for a casting operation, resulting in increased cost. Moreover, even such a gas injection-type nozzle has difficulty in fully preventing the deposit formation.
An upper nozzle has been widely used, for example, in the following two configurations: one consisting of a reverse taper region formed on an upper (upstream) side of the upper nozzle and a straight region formed on a lower (downstream) side of the upper nozzle (see FIG. 12(a)); and the other having an arc-shaped region continuously extending from the reverse taper region and the straight region (see FIG. 13(a)). In each of FIGS. 2 to 13, the diagram (a) shows an upper nozzle which is installed in a sliding nozzle unit (hereinafter referred to as “SN unit”), wherein a region downward (downstream) of the one-dot chain line is a bore of an upper plate, and a region downward of a position where two bores are out of alignment is a bore of an intermediate plate or a lower plate.
As a result of calculation of a distribution of pressures to be applied to a wall surface of a bore (bore surface) of an upper nozzle (length: 230 mm) having the configuration illustrated in FIG. 12(a) during flowing of molten steel through the bore, it was verified that the pressure is rapidly changed in a region beyond a position (180 mm from an upper (upstream) end of the bore) where the bore surface is changed from a reverse taper configuration to a straight configuration, as indicated by the dotted line in FIG. 12(b).
Further, as a result of calculation of a distribution of pressures to be applied to a wall surface of a bore (bore surface) of an upper nozzle (length: 230 mm) having the configuration illustrated in FIG. 13(a) during flowing of molten steel through the bore, it was verified that the pressure is changed in an arc curve, i.e., a pressure change is not constant, as shown in FIG. 13(b), although a rapid pressure change is suppressed as compared with the upper nozzle illustrated in FIG. 12(a) which has a bore surface changed from a reverse taper configuration to a straight configuration. In each of FIGS. 2 to 13, a region rightward of the one-dot chain line in the graph (b) shows pressures to be applied to a wall surface of the bore (bore surface) of the upper plate.
The rapid pressure change and the arc-curved pressure change is caused by a phenomenon that a molten steel flow is changed as the bore surface is changed from the reverse taper configuration to the straight configuration. Further, in a swirling nozzle adapted to intentionally change a molten steel flow, a deposit is observed around a position where the molten steel flow is changed. Thus, it is considered that a deposit inside the bore of the upper nozzle can be suppressed by creating a smooth molten steel flow, i.e., a molten steel flow having an approximately constant change in pressure on the bore surface.
As a technique of stabilizing a molten steel flow, there has been proposed an invention relating to a configuration of a bore of a tapping tube for a converter (see, for example, the following Patent Document 3).                [Patent Document 1] JP 2007-90423A        [Patent Document 2] JP 2005-279729A        [Patent Document 3] JP 2008-501854A        